The present invention relates to a method for controlling the torque of a synchronous machine supplied by a frequency converter. In this method, starting from measured variables representing the state of the machine, the direct axis flux component, the quadrature axis flux component and the pole angle for the machine are determined, and set values are produced for the flux, excitation current and current controlling the torque.
The frequency-controlled drive of a synchronous machine requires a rapidly adjustable frequency converter as well as control dependent on the pole angle and the position of the rotor and ultimately adjusting the main flux. The oldest of such systems from the 1960's are so called characteristic curve-controlled systems (Stemmler, H., Antriebssystem and elektronische Regeleinrichtung der getriebelosen Rohrmuhle. Brown Boveri Mitteilungen 57, H. 73, pp. 121-129 (1970)) wherein stator and rotor magnets are controlled by adjusting, by means of characteristic curves stored in function generators, the magnetic currents and the angles therebetween to such values as allow for a drive with a constant flux and the desired power ratio with all permissible loads. Such rotor oriented drives operating in the polar coordinates already represented a rather successful control principle, but the adjustment dynamics achieved therewith was not sufficient in view of sophisticated drives (e.g. rolling mill drives), since the control was specifically based on the characteristic curves of the constant state of the synchronous machine.
The controllability of the synchronous machine was improved, when starting from the beginning of the 1970's the so called vector-controlled field oriented control procedure was applied (Blaschke, F., Das Verfahren der Feldorientierung zur Regelung der Drehfeldmaschine. TU Braunschweig, 1974 (dissertation)). In flux orientation, the stator magnet is controlled either in the direction of the main flux (dynamic flux control) or perpendicularly to the main flux (torque control/adjustment on the torque axis). Rotor current control dependent on the stator current is often added to the flux-oriented control procedure, but the method and precision of calculating the set value for the current in relation to the set values for the stator currents varies in different applications (Buhler, H., Einfuhrung in die Theorie geregelter Drehstromantriebe. Birkhauser Verlag Basel und Stuttgart, 1977).
The basic disadvantage of the known methods resides in that they allow the pole angle to be formed freely. Furthermore, they utilize in a static synchronous machine not furnished with a damper winding the so called synchronizing torque (torque variation relative to variation of pole angle) also in dynamic torque control. With large pole angles, the synchronizing torque is decreased, which may result in the "pull out" of the drive into the pure reactive current state when the pole angle exceeds the value 90.degree.. The practicability of these methods has been improved by means of a damper winding partially separating the dynamic torque control and the pole angle control from one another.